In the following number-pairs , the second number is obtained by applying certain mathematical operations to the first number. Which number should replace X and Y so that the pattern followed by the two numbers on the left side of :: is same as that on the right side of :: ?

(NOTE : Operations should be performed on the whole numbers, without breaking down the numbers into its constituent digits. E.g 13 - Operations on 13 such as adding/subtracting/multiplying etc. to 13 can be performed. Breaking down 13 into 1 and 3 and then performing mathematical operations on 1 and 3 is not allowed.)

X : 679 :: 864 : Y

Given: 

X : 679 :: 864 : Y

From the options - 

let the calculate the difference between X , Y and Number - 

825 : 679  Difference 825 - 679 = 146

864 : 718 Difference 864 - 718 = 146

let the calculate the difference between X , Y and Number - 

812 : 679  Difference 812 - 679 = 133

864 : 756 Difference 864 - 756 = 108 

Difference not equal. 

let the calculate the difference between X , Y and Number - 

834 : 679  Difference 834 - 679 = 155

864 : 704  Difference 864 - 704 = 160

Difference not equal. 

let the calculate the difference between X , Y and Number - 

898 : 679  Difference 898 - 679 = 219

864 : 649 Difference 864 - 649 = 215

Difference not equal. 

Thus, the correct answer is (a).